On a line in operation, a track is divided into sections of 400 to 2500 m, on each of which the trains move at a speed depending on a signal exhibited at the entrance to the section, usually the colour of a three-colour traffic light, which is green, if the section is free, orange, if the next section is occupied by a train—the light at the entrance of this next section is therefore red-, and red, if the preceding section is occupied.
For a train to be able to run on a section at the speed of the line, that is to say run “in section”, it is preferred that at least the two sections that follow this section are free of any other train. It is in these conditions that the signal is green at the entrance of the section in question.
The presence of a train on a section is detected by a track circuit which controls the signalling elements that have just been evoked.
Fitting a line under construction with such a signalling system is out of the question. Its cost would be prohibitive.
Up to now, the signalling for the movement of work trains on the tracks of a railway line under construction has been provided in empirical fashion by the work-site personnel with the aid of signboards and other panels and of radio and telephone means. The major disadvantage of such an empirical solution, not to mention the requirement for significant staffing, is that it limits the speed of the trains on the tracks, the drivers being obliged to drive “in work mode”, that is to say running by sight.